1. Field of the Invention
This invention concerns microfluidic devices comprising porous monolithic polymer suitable for extraction, preconcentration, concentration and mixing of fluids. In particular, the invention concerns microfluidic devices comprising monolithic polymers prepared by in situ initiated polymerization of monomers within microchannels of a microfluidic device and their use for solid phase extraction, preconcentration and mixing.
2. Background and the Related Disclosures
The interest in microfabricated devices designed for micro-total analytical systems (μ-TAS) is growing rapidly.
A number of applications for analytical microchips in areas such as enzymatic analysis, polymerase chain reaction (PCR), immunoassay, DNA sequencing, hybridization, mapping, isoelectric focusing, capillary zone electrophoresis, as well as capillary electrochromatography have already been reported. The first products involving microfluidic chip designed for the gel electrophoretic analysis of biopolymers are now commercially available, for example, from Agilent Technologies. These products were shown to compete successfully with classical techniques, such as polyacrylamide gel electrophoresis (PAGE).
Microanalytical systems with more complex architectures have also been reported, for example, in Electrophoresis, 21:3931-3951 (2000); Anal. Chem., 70:232-236 (1998); Sens. Actuat. B-Chem., 72:273-282 (2001); Anal. Chem., 69:3646-3649 (1997); and Science, 273:205-211 (1996).
Despite the usefulness of microfluidic chips in a variety of applications, some problems with their use still persist. For example, almost all reported microfluidic chips feature open channel architecture where the surface to volume ratio is rather small. This presents a serious problem in applications such as chromatographic separations, heterogeneous catalysis, and solid phase extraction that rely on interactions with a solid surface. Since the only solid surface within these chips is the channel wall, the chip can handle only minute amounts of compounds.
The issue of surface area in the macroscopic devices can be solved by packing them with porous particles that significantly increase the available surface area and also enable the introduction of specific chemistries into the device. Early attempts to pack a microchip channel with beads were less successful (Anal. Meth. Instrum., 2:74 (1995)). J. Anal. Chem., 72:585 (2000), for example, describes a device for solid phase extraction by packing octadecyl silica beads from a side channel into a specifically designed cavity of the microchip.
A few reports have dealt with attempts to enhance the limited surface area in channels of a microchip without packing. For example, J. Chromatogr., 853:257 (1999), reports fabricated microchip channels containing arrays of ordered tetragonal posts. Electrokinetic preconcentration of DNA from dilute samples using porous silicate membrane incorporated into the microchip has also been reported. Selecting the pore size which permits passage of electric current but prevents passage of large molecules has enables their concentration (Anal. Chem., 71:1815 (1999)).
Another technique enabling an increase in the concentration of desired compounds involves sample stacking, a technique successfully used in capillary electrophoresis. (Anal. Chem., 70:1893 (1998). However, this approach is only practical for electrodriven systems. In contrast, solid-phase extraction (SPE) is a more general method since it enabled handling of large sample volumes regardless of the method used to ensure sample flow (Anal. Chem., 72:4122 (2000)).
In the early 1990s, macroscopic rigid porous monoliths prepared in situ by a thermally initiated polymerization process were introduced and are disclosed, for example, in U.S. Pat. Nos. 5,334,310; 5,453,185 and 5,593,729. Their use has been described in a number of applications including HPLC and CEC of small molecules, chiral compounds, proteins, peptides, and nucleic acids (Anal. Chem., 72:4614 (2000)). The monolithic technology has also been successfully applied to the preparation of devices for scavenging undesired compounds from solutions and for SPE (J. Comb. Chem., 3:216 (2001)); Chem. Mater., 10:4072 (1998)) and good control over both porous properties and surface chemistry of the monolithic polymers was achieved.
Yet another of the persisting problems of microfluidic chip technologies is also the lack of efficient mixing within the channels. The simplest solution to the mixing problem is the use of so called T-piece. For example, parallel mixing has been demonstrated at various mixing ratios using a series of T-intersections (Anal. Chem., 71:5165 (1999)). However, this does not solve the problem of the lack of turbulences and using this approach, mixing within a substantial channel length can only be achieved at low flow velocities. An increase in mixing efficiency has been observed after dividing both phases into a larger number of small parallel channels, mixing the streams in each of these channels, and recombining the sub-streams again into a single channel. In this approach, enhancement of the mixing results from the increase in contact area between the two phases (Anal. Commun., 36:213 (1999)).
A somewhat similar strategy involves splitting the streams in an array of small laminae followed by their recombination in a triangular chamber (Sens. Actuat. B-Chem, 72:273 (2001)). A picoliter-volume mixer with a wave-like architecture that includes multiple intersecting channels of varying lengths having a bimodal width distribution was recently introduced (Anal, Chem., 73:1942 (2001)). This approach enhanced both lateral and longitudinal mixing and the device performed efficiently.
Still another implementation includes a three-dimensional serpentine microchannel design with a “C-shaped” repeating unit fabricated in a silicon wafer (J. Microelectromechanical Systems, 9:190 (2000)).
The manufacture of all of the micromixers discussed above involves the use of typical microfabrication techniques. Both, rather complex designs and very small features, can easily be fabricated in substrates such as glass or silicon. However, production of devices with very fine features using polymer-based substrates and the use of fabrication techniques such as hot embossing and injection molding is still lagging.
Previously, these inventors demonstrated that the porous properties of the monolithic polymers can be controlled within a broad range by pore size. However, the thermally initiated free radical polymerization used originally was not well suited for the preparation of monolithic structures within microdevices since selective heating of specific areas of the microchip to locate the monolith strictly within the assigned space was difficult to achieve. This problem was subsequently overcome by the development of UV initiated polymerization processes, described in Electrophoresis, 21:120 (2000), which is similar to the photolithographic patterning used in microelectronics. Photopolymerization enables the formation of monoliths only within a specified space. The polymerization reaction is strictly confined within the areas exposed through a mask, while no polymerization occurs in unexposed zones.
It is therefore a primary objective of the current invention to provide a microfluidic device comprising a microchannel filled with a polymerization mixture that is solidified by an in situ initiated polymerization into a monolithic polymer with a specific properties permitting a solid phase extraction, concentration, preconcentration and mixing of fluids.
All patents, patent applications and publications cited herein are hereby incorporated by reference.